Digitally encoded information is often stored or processed using an addressing scheme that electronically accesses individual data storage sites arranged in a solid-state digital array. Japanese Laid Open Patent Application No. 217396/89 and its counterpart U.S. Pat. Nos. 4,896,149 and 5,077,553, which are assigned to the assignee of the present application, disclose an addressing apparatus of this type in which row addressing is achieved by means of an ionizable gas. FIG. 3 depicts a prior art flat panel video display that incorporates such an apparatus.
The flat panel display apparatus 10 includes electrically non-conductive, optically transparent first and second substrates 12 and 14 positioned face-to-face to each other. Multiple conductors extending in parallel to one another in a first direction on the inner surface of the first substrate 12 form column electrodes 16 for receiving data drive signals from a data drive circuit (not shown). Multiple channels 18 are formed in the inner surface of the second substrate 14 so that they extend parallel to one another in a second direction transverse to the first direction. A reference potential row electrode 20 and a strobe row electrode 22 electrically isolated from each other extend along the length of the interior of each channel. The reference potential row electrodes 20 are commonly grounded on one side of the channels, and each of the strobe row electrodes 22 receives a data strobe signal from a data strobe circuit on the other side of the channels. Each of the channels 18 is filled with an ionizable gas, such as helium.
A layer 24 of electro-optic material, such as a liquid crystal material, and a layer 26 of a dielectric material are positioned between the first and second substrates 12 and 14. The layer 26 of dielectric material covering the channels 18 forms a barrier between the liquid crystal material layer 24 and the ionizable gas. The size of display elements 28 defined by the overlapping areas of the column electrodes 16 and the channels 18. The ionizable gas functions as an electrical switch that changes between a conducting or plasma state and a nonconducting or nonionized state in response to the voltage of the data strobe signal.
Whenever the gas is in a conducting state, the voltage of the data drive signal is developed across the liquid crystal material in a region spatially aligned with the region of ionized gas. When display apparatus 10 receives light propagating from an external source, the data drive signal on a column electrode 16 determines the luminance of a display image. Whenever the gas changes to the non-conducting state, the voltage of the data drive signal developed across the liquid crystal material is held for a predetermined time. Such addressing apparatus of the flat panel display 10 is referred to as a plasma addressing apparatus. The plasma addressing apparatus provides an improved yield for a flat panel display having a layer display element density because it does not rely on two- or three-terminal semiconductor devices.
FIG. 4 shows a part of the second substrate 14 in detail. The channel 18 is formed by a flat surface of the second substrate 14 and opposite side walls 32 of adjacent projections 30. When the data strobe signal having a predetermined voltage is applied to the strobe row electrode 22, the gas between the reference row electrode 20 and the strobe row electrode 22 changes to the conducting state. Then, the portion of liquid crystal material located in an area defined by the space between the row electrodes 20 and 22 and the width of the row electrode 16 acts as a display element. It is desirable that the space between the row electrodes 20 and 22 be close to the width of the channel 18 to provide narrow gap between the display elements. Thus, the row electrodes are deposited on the side wall 32 of the projection 30. The row electrodes 20 and 22 pass along the steep end wall 34 at both ends of the projection 30 and extend out of the channel 18 to receive the reference potential and the data strobe signal, respectively.
Proper operation of display apparatus 10 depends in part on the structural quality of the electrodes that energize the ionizable gas contained in the channels. The row electrodes 20 and 22 are formed using a conventional photolithographic technique. In the manufacturing process, a metal film of conductive material, such as nickel, is deposited over the inner surface of the second substrate 14 having the channels 18. Then, a photoresist layer is formed over the metal layer and exposed to incident light passing through a pattern having openings corresponding to the row electrodes 20 and 22 to be formed. By developing the photoresist, the photoresist material in the area where the row electrodes 20 and 22 are not formed is removed. The row electrodes 20 and 22 are formed by etching the exposed metal film.
There is, however, a problem stemming from the fact that the end wall 34 is much steeper than the remaining area. The thickness of the photoresist on the end wall 34 is, therefore, much thinner than that of the photoresist in the remaining area. When the developing process is stopped at the time the light-unexposed photoresist on the end wall 34 is removed, the light-unexposed photoresist in the remaining area is left because it is not developed sufficiently. The metal film under the left light-unexposed photoresist is not exposed to the etching agent so that it is left as an undesirable metal film after etching. Thus, there is a possibility that the undesirable metal film short-circuits between the row electrodes 20 and 22. On the other hand, when the developing process is stopped at the time the light-unexposed photoresist in the remaining area is removed, the photoresist on the end wall 34 is developed to excess so that the exposed photoresist is partly removed. Thus, there is a possibility that the row electrodes 20 and 22 have open portions. The short-circuit and the open portions of the row electrodes 20 and 22 prevent the gas in the channel 18 from being placed in the conducting state.
An object of the present invention is, therefore, to provide electrode contacts of more consistent structural quality such that addressing apparatus incorporating these electrodes may manufactured at higher yield.